Heater control question

freesaints · 09-05-2021, 03:10 PM

Does anyone know what this wire controls? It's #10 on the diagram. I'm trying to see if I can operate the heater knob without that wire. Right now, I only use the heater sparingly and when I do, it's usually to crank it to 9 or 10 and let the hot air seep in to the cabin through the open tube in both footwells (I've removed the footwell blower motors and hoses).

freesaints · 09-05-2021, 03:14 PM

Here's the wire/connector in question and the diagram:

garment · garment's Garage

What year car? Do you have a working blower in your engine bay?

freesaints · 09-05-2021, 03:25 PM

87 930. Not sure if the blower is working. I have heat exchangers and if I turn the dial to 9 or 10, I get a decent amount of heat into the cabin. Plus the requisite oil smell.

cpa · cpa's Garage

That is the feed for the cabin thermistor it tells the controller the temp in the cabin. If all you want is heat it will probably work without connecting it.

freesaints · 09-05-2021, 04:12 PM

Thanks guys, but still a little confused as to why I'd need that. I have an aftermarket AC unit that I control with the knobs in the center console. I also get heat with the heater knob above the parking brake as mentioned. I've deleted the HVAC controls from the dash so I'm not sure what needing to know the temp in the cabin is for. Does it affect how much heat is coming through the engine bay, or how much AC to send into the cabin? I'd assume those are manually controlled by the dials.

garment · garment's Garage

More likely than not, your heater blower is kaput. I had the same symptoms as you. I swapped for a good unit and now I get a range of heated air velocity through the range of auto-heat knob numbers. You may want to also examine the autoheat potentiometer - Bentley has a diagnostic procedure.

freesaints · 09-05-2021, 04:58 PM

Thanks garment, that may be the case. If I were to diagnose the heater blower and got it functional, I'm still wondering what the cabin thermistor does in relation to that. For instance, if I rotate the dial (with a functional blower motor), the volume of hot air into the cabin increases. Makes sense. But I'm still wondering if I need that connection or if I could go without it and still get that hot air into the cabin. Or asked another way, what would happen if I remove that wire with my current setup?

garment · garment's Garage

Don’t remove it. The purpose of the sensor between the visors is to detect the temp in the cabin and adjust the heat appropriately - hence the (relatively) sophisticated “autoheat”. Solve for the blower motor first.

Quote:

Originally Posted by freesaints

Thanks garment, that may be the case. If I were to diagnose the heater blower and got it functional, I'm still wondering what the cabin thermistor does in relation to that. For instance, if I rotate the dial (with a functional blower motor), the volume of hot air into the cabin increases. Makes sense. But I'm still wondering if I need that connection or if I could go without it and still get that hot air into the cabin. Or asked another way, what would happen if I remove that wire with my current setup?

pete3799 · pete3799's Garage

I believe that if either of the thermistors are disconnected (or shorted out) then the auto heat defaults to full on in any dial position. If both thermistors are working they regulate the opening of the flapper valves. If you have a lever next to the auto heat unit you can disconnect the rod (inside the auto heat control) that goes to the servo and use the lever to control the amount of heat you want.